The invention concerns improvements in or relating to seals, and has particular, though not exclusive, reference to seals of the kind employed in sealing the annular gap between a rotary shaft and a housing through which such shaft extends.
The profiles of present day elastomer seals bonded to an "L" section metal shell are a compromise between optimum design and production needs, an essential requirement being a "V" shaped sealing edge radially loaded in contact with the rotary shaft by a close-coiled circular section spring.
Synthetic rubber materials are friction producing, and thus gives rise to a temperature at the sealing edge which is sufficient, in some instances, to give excessive shaft and sealing edge wear.
The oil film thickness under a radial seal lip is of the order of 5 to 20 microns, this giving a meniscus on the sealing surface with no visible fluid leakage to the air side. Over a period of time the sealing surface increases in area with a consequent increase of the meniscus ultimately to a condition where visible leakage occurs.
To counteract this leakage, hydrodynamic fluid return formations of various kinds have been proposed, but although such formations are initially effective, their effectiveness diminishes with increasing wear. In out prior United Kingdom Pat. Nos. 1,382,281 and 1,382,478 there are disclosed hydrodynamic fluid return formations which function when a seal is worn sufficiently to bring the apices of the hydrodynamic fluid return formation into use, the concept embodied in such fluid return formations representing a significant advance over prior art proposals.
With increasing labour costs the demand is for extended seal life, in many instances in excess of the product material remaining consistent in hardness and flexibility, notwithstanding that the use of inhibitors in lubricating oil in many instances results in degradation of the elastomer, and consequential shortening of the effective seal life.
Leather seals have a longer life potential than synthetic rubber, providing the speed and temperature are within the limits for the material, and such seals will function satisfactorily on shafts with a directional lay and roughness unacceptable to synthetic rubber elements. The comparatively wide band of sealing surface of a leather seal resists the ingress of dirt, and other contaminants, but the profile of such seal cannot be simulated satisfactorily in synthetic rubber because of the high torque arising from such a wide band and the resultant seal head distortion and inevitable increase in temperature.
It is known, for example, from U.S. Pat. No. 2,804,325 to provide a polytetrafluoroethylene sleeve through which the pressure of a garter spring, which is located around a frusto-conical elastomeric support, is transmitted to the shaft surface to be sealed. However in the arrangement disclosed the elastomeric support bears over a large area onto the sleeve and in consequence the sleeve bears over a large area onto the shaft surface. In order to provide sufficient bearing pressure to prevent leakage a strong garter spring is required. Due to the large bearing area high wear and running temperatures will result, thus necessitating a relatively thick and inflexible sleeve if frequent replacement is to be avoided. Such a sleeve will not readily follow eccentricities of the shaft and in consequence leakage of the seal can occur. A hydrodynamic wind-back arrangement in the form of a spiral groove is provided, but even under high spring loadings leakage of the fluid through the seal can occur when the shaft is static.